The green liquor that is a process liquor in the causticisation process consists principally of sodium carbonate. The green liquor is caused to react during the process with quicklime, whereby calcium carbonate (lime mud) is formed, together with sodium hydroxide (white liquor). The white liquor is filtered in tube filters or disc filters, where these filters, in particular the latter, are normally of pressurised type. The white liquor obtained is reused in the sulphate process for the cooking of pulp wood chips. The lime mud obtained from the filtration is finally burnt in order to reform quicklime.
As is described in Chemical Pulping, Book 6B, ISBN 952-5216-06-3 in the section White liquor preparation (pp. B133-B202), the lime mud is conventionally treated after the white liquor filter such that it is formed into a slurry with a consistency of 35-40% and it is stored in storage tanks in order to ensure a steady supply of lime mud to the lime kiln. The lime mud that has been slurried is pumped from these storage tanks to a lime mud dewaterer that is arranged just before the lime kiln, where dewatering of the lime mud slurry takes place to give a level of dry matter content of 65-90%. The dewatering normally takes place in atmospheric drum filters and the lime mud that is scraped from the dewatering filter is transported on transport belts to the lime kiln. This entails a number of unnecessary operations and a certain amount of unnecessary equipment in the form of sludging arrangements, storage tanks and dewaterers.
In continuously operating filtration arrangements for liquid suspensions such as, for example, those described in SE-C,463 771 (equivalent to U.S. Pat. No. 4,929,355), a filter cake in the form of lime mud on a filter medium is formed. This lime mud is scraped off with scrapers (known as “knives”) and falls down into a reception chute, where further dilution with dilution liquid is carried out in order for it to be possible to transport the lime mud onwards. An output screw is shown in the said SE-C 463 771, in one embodiment, directly after the reception chute, which screw feeds the mud onwards to a sluice. The sluice is, in a second displayed embodiment, in the form of a storage tank in which the level is monitored, in which tank a mechanical stirrer is placed.
A corresponding solution with storage tank and mechanical mixer is shown in U.S. Pat. No. 5,151,176. Storage tanks and mechanical stirrers are used also in other applications with tube filters, see U.S. Pat. No. 4,264,445, in order to deal with the filter cake that is removed from a pressurised tube filter. It has been considered that the bulky storage tank and the mechanical stirrer arranged within it are necessary in order to be able to retain the lime mud well-mixed with the added dilution liquid and to prevent sedimentation of the lime mud.
An alternative system from Larox OY is shown in WO 97/22752 in which the lime mud is separated from a white liquor filter arranged directly after the causticisation vessel. The lime mud is here stored in a special intermediate silo that has been designed such that it is possible to feed the lime mud in dry state directly to the lime kiln. It is possible to avoid with this type of system the formation of an intermediate slurry from the mud, and there is no dewaterer for the lime mud before the dry lime mud is fed into the lime kiln.
It is thus possible to handle the lime mud after its separation from a mud-containing liquid mixture in its slurried form as specified by conventional technology, or to handle it in its dry state, with a level of dry matter content of 70-80%. The conventional technology will be unnecessarily expensive since several additional stages, such as intermediate storage of the slurried lime mud in silos with continuous stirring and the subsequent dewaterers, are required.
Handling dry lime mud involves very serious problems since the lime mud produces dust and gives rise to environmental problems in the recovery process, and it is also highly wearing for processing equipment since the dry lime mud behaves as a grinding powder. The wear means that handling equipment for dry lime mud from pressurised filtering processes cannot be constructed to use pressurised sluices having mechanical sealing arrangements with small tolerances since these parts wear rapidly. Furthermore, dry lime mud is difficult to handle since the mud has a tendency to clog storage silos and pipe systems. The dry lime mud has for this reason often been handled in open systems that feed the mud to the lime kiln on transport belts or similar. The systems, furthermore, must be able to feed the lime mud to the lime kiln in an even flow, since the kiln can easily damage the ceramic lining by burning, if the kiln is not evenly filled throughout its extent. It is normal to counteract a disturbance in the feed of lime mud by reducing the power of the burner in the lime kiln in order to prevent a poorly filled section of the lime kiln being exposed to a heat load that is too high.
The invention relates to the handling of a finely granulated material, by which is denoted a material for which the particle size is less than 1,000 micrometers (<0.1 cm), preferably less than 100 micrometers, where the material forms a tightly packed structure with a very high pressure differential across a bed that is formed from packing such material. The feed out of dry lime mud is preferentially concerned, the behaviour of which is similar to that of cement powder or flour, and where this finely granulated material forms a compact mass when a bed is constructed.
The invention is based on the insight that a specific tube valve can be used as outlet sluice from a pressurised process from which finely granulated material is fed out, which material otherwise has a tendency to clog the outlet. Through holding the finely granulated material as a unified column, a functional pressure lock is formed in order to ensure that the pressurised process can maintain its working pressure with minimal losses caused by leakage through the outlet.
The purpose of the invention in the form of the system is to provide an output system for finely granulated material from a pressurised process where:                Pressure losses through the outlet are minimised        Tendencies for the formation of plugs can be monitored and exploited        It is possible to establish a pressure lock without forming the finely granulated material into a slurry (it is not necessary to establish a liquid trap).        
A further purpose is to make it possible to feed the dry lime mud directly to the lime kiln during the application for cleaning lime mud.
Further characteristics and aspects and advantages of the invention are made clear by the following description of some putative embodiments.